Frequency selective system



ug. l5, 3.939. M C, TOURNIER 2,169,304

n FREQUENCY SELECTIVE SYSTEM Filed June 23, 1957 FIG. 2

A 7' TOR/VE V 4Patented Aug. l5, 1939 UNITEB STATES FREQUENCY SELECTIVE SYSTEM Marcel Charles Tournier, Paris, France, assgnor to Western Electric Company, Incorporated,

New York, N. Y., a corporation of New York Application June 2s, 1937, seriaiNo. 149.89 e In France July 18, 1936` 3 Claims.

This invention relates to wave selecting systems and more particularly to systems for the selection and separation of waves propagated in elastic media.

5 In accordance with the invention waves are excited in an elastic medium by an array of point or line sources so spaced and phased relatively to each other as to produce a denite space interference pattern in the medium. Because of the interference eiects, waves of different frequencies are propagated in diierent directions in the medium and may be selectively received` by means of receiving devices placed at appropriate points in the medium.

The method of selection is analogous to the separation of the components of `light by means of a diiraction grating, which, as is well known, eiects a directional separation of the light components of different colors or wave-lengths. It

is also analogous in certain respects to the directional separation of electromagnetic waves in radio communication by means of antenna arrays.

The invention is particularly adapted for the selection of high frequency elastic waves having lengths of the order of one or two centimeters or less. These may, for example, be compressional elastic waves of about 100,000 cycles per second or greater propagated in water or other liquid medium. Excitation of the waves in the medium may be effected electrically by aperiodic piezoelectric quartz plates and similar quartz plates either aperiodic or tuned may be used for receiving the waves and converting them into electrical oscillations. With such arrangement, the

of electric waves, for example, intermediate frequency waves in a superheterodyne radio receiver.

In the detailed description which follows illustrative examples of arrangements for carrying out the invention are described. It will be understood, however, that the invention is not restricted to these particular embodiments, but only in accordance with the scope of the appended claims.

Of the appended drawing, Fig. 1 is explanatory of the principles of the invention; Fig. 2 shows a wave translating device ordance with the invention;

Fig. 3 illustrates a modied form of wave translating device;

Fig. 4 shows a selective system in accordance with the invention; and

invention provides for the mechanical selectionl in ac- Fig. 5 shows a. modified form of selective system of the invention.

a Referring to Fig. 1, A and B represent the traces of linear elements, perpendicular to the plane of the paper, giving rise tocylindrical lon- 5 gitudinal waves in an elastic medium. Each element may be considered as a wave source, the plane of the wave propagation being that of the paper. Let the distance between A and B be denoted by a and consider the propagation of waves l0 of length A in a direction at the angle a to the line joining A and B.l It is assumed that the wave sources are in phase.

If a plane receiving element be placed at a distance D, which is large compared with a, in the direction a, the difference in step between thewaves received from the two sources will be equal to a cos a. Maximum reception will obtain when this difference is equal to an integral number of whole wave-lengths or when nda: a sin ada, (2)

from which, with the help of Equation 1, is obtained Equation 3 indicates that, to obtain a large angu lar dispersion of agroup of Waves diiering slightly from each other in frequency, it is desirable that the angle a should be small. This requires that the spacing of the source elements should be such that the central waves of the group reinforce each other in a direction well removed' from the normal to the plane of the elements, for example at an angle of 45 degrees. Thus, if 50 the transmitting medium be water, for which the velocity of propagation is 1400 meters per second, and if the central wave of the group has a frequency of kilocycles, the central wavelength is Licentimeters and the rst order specvlliV in Hg. 2. It comprises a quartz plate I having` electrodes II and I2 preferably in the form. of lo metal foil or iilm attached to its opposite surfaces and connected to an electrical oscillation source G. The quartz plate is mounted behind and parallel to a slotted plate I3 and enclosed in a housing Il. Preferably the plate I3 and the housing Il are of vibration absorbing material or are covered on their interior surfaces with vibration absorbing material such as lead. The quartz vibrator should be cut with respect to the crystal axes in such manner that it vibrates primarily in the direction of the thickness of the plate.V The slotted plate I3, which functions in the manner of a diiraction grating, may have Il or more equidistant slotsl the eil'ect of the larger number of slots being to increase the directivity of the Wave interference effects and the sharpness of the amplitude maxima, while still maintaining the relationship between the angular directions of the maxima and the wavelength, The quartz plate I0 should extend somewhat beyond the length of the space occupied by the slots in plate I3 and is preferably dimensioned. otherwise to be substantially free from mechanical resonances in the neighborhood of its operating range. DWhen subject to oscillatory voltages from source G, the quartz plate sets up compressional waves in the surrounding medium and these advance with a substantially plane Wave front towards the grating E. Each slot then becomes a secondary wave source and, because of 40 the plane character of the exciting wave front,

all slots generate waves in like phase.

For a central frequency of |00 kilocycles the Y spacing of the slots should be about 2.0 centimeters when the medium is Water, and the width of the slots about l millimeter or less. A 1 artz plate centimeters or more in length Vgould be required, vFor higher frequencies the dimensions may be correspondingly reduced. If Eater is used as a medium, it vshould preferably be dis- 50 tilled or otherwise purified to provide high electrical insulation. Alternatively other insulating fluids may be used, those having low viscosity being preferred. v

The eiiiciency of the device can be increased 55 by halving the separation of the slots and so arranging that the alternate slots are excited in opposite phases. Under this condition the diiler- Vence of the path lengths from alternate slots in the direction of a maximum is only one-half 00 Athe wave-length, but because of the phase oppositionrof the excitation the waves combine additively at a receivingpoint. An arrangement for effecting this is shown diagrammatically in Fig. 3. A plurality of separate similar quartz plates q1 0l to qm are supported side by side in a plane, each plate having individual electrodes. I'he electrodes of the alternate plates are reversed in polarity by their connections to the generator G as indicated in the igure. In front of the plates .70. and parallel to the plane Vthereof is placed a grating Il having a slot opposite to each of the quartz plates. 'I'he plates may be enclosed in a housing such as shown at Il in Fig, 2. For a given wave-length the separation of the slots in this case is only one-half as great as in the device of Fig, 2. 'I'he overall length of the device may therefore be cut in half or, alternatively, twice as many slots may be used in a given overall length as compared with the system of Fig. 2.

A complete selecting system in accordance with the invention is shown in Fig. 4, this system being suitable for the separation of three frequencies or channels. In this gure C is a cell or tank containing the. propagation medium M, in which is disposed a. translating device T, of the type shcwnin Fig 2, comprising a quartz plate QA, a grating E and a wave absorbing housing PA. The electrodes of the quartz plate are connected through high frequency amplifier AHP to an electrical wave source which is not shown, but; which may be oi' any character. In front of, or below, the grating E and at a suitable distance therefrom receiving quartz plates r1 to r6 are mounted on a wave absorbing screen P, each platev being disposed in an angular direction to the normal from the plate E in accordance with the frequency it is intended to receive. Normal to the screen is placed a block of absorbent material, such as sponge rubber to receive and absorb the undiracted waves. The electrodes of receiving quartz plates r1 are shown connected through a receiving high frequency amplifier RHF to a detecting or utilization device D. The other plates. may be likewise -connected to individual receiving devices, but for the sake of clearness these connections are omitted from the diaphragm,

To prevent interfering .reflections from its walls, the tank C should preferably be lined with vibration absorbing material, such as sponge rubber. Alternatively the tank may be made large in comparison with the dimensions of the apparatus contained in it, but the use of the wave absorbent lining permits the dimensions to be held to convenient limits.

lThe receiving quartzes may be aperiodic or they may be tuned to respond only to the frequency to be selected. Tuning of the plates r4 to r6 is shown by the addition of metallic loading blocks a4, as, and ae, of suitable thickness. Other tuning means may, of course be used, for example the suitable proportioning of the quartz plate dimension in accordance with known principles.

The width of the receiving quartz plates determines the frequency range received thereby in accordance with Equation 3. For example if a wave group having a central frequency of 100 kilocycles is received at an angle of degrees, a receiving quartz plate covering an angular range of 3 degrees will receive `a band 5 kilocycles wide. In combinatih-lwith the sharp .directivity provided by the multiplicity of slots in the transmitting device, the limitation of the dimensions of the receiving plate provides for a sharply discriminating band selectivity.

` Increased selectivity may be obtained by the modied arrangement illustrated in Fig. 5, 'in which the piezoelectric wave source with its diftracting screen E is placed at the focus of `a convex parabolic reector R. Wave groups of different central frequencies are reflected from the reector withincreased divergence and are received upon a second slotted screen E2 beside the slots of which are disposed the receiving elements r1 to-n. 'Ihe increased divergence of the waves produced bythe reflector and the limiting action of the slots in the second screen provide for extremely' sharp frequency selectivlty.

What is claimed is:

1. A frequency selective transmitting system comprising an elastic uid medium, a source of plane compressional waves of a plurality of different frequencies disposed vin said medium, a diiraction grating disposed in said medium adjacent said wave source, said grating comprising a slotted plate having a plurality of narrow equally spaced parallel slots and being positioned with respect to said source so that waves therefrom arrive at the several slots at phases related by integral half periods of the wave vibrations, and a plurality of Wave responsive devices disposed in said medium beyond said plate in directions at diierent finite angles to the normal to said plate and in a. plane perpendicular to 'the axes-of said slots, the angular positions of saidreceiving devices being determined respectively in accordance with the diierent frequencies of the oscillations from said source.

2. A frequency selective system in accordance with claim 1 in which the said Wave responsive devices comprise piezoelectric crystal elements on the opposite surfaces of said plate and anA electrical oscillation source connected to said -electrodes, and in which the said grating comprises a at slotted plate disposed in a plano parallel to the surface of said quartz plate, the slotted area of said grating being substantially coextensve with said piezoelectric plate.

MARCEL CHARLES TOURNIER.

tuned to the respective frequencies received, elec- 

